Victoria’s chief health officer Charles Guest released the updated warning on Tuesday, noting the link between thunderstorms and asthma epidemics due to the fragmentation of pollens and the disturbance of other environmental allergens, especially during the grass flowering season.

“Epidemic thunderstorm asthma events are infrequent and difficult to predict. However, they occur during hay fever season (October to mid-December),” the warning reads.

“It is therefore important that people are aware of this phenomenon and act on any respiratory symptoms that develop.”

The outbreak triggered more than 1900 calls to Ambulance Victoria last Monday night and Tuesday morning, and hospitals across Melbourne were inundated with 8500 patients suffering from respiratory conditions.